The present invention relates to a photopolymerizable resin composition or, more particularly, to a photopolymerizable resin composition suitable for patterning by the pattern-wise exposure to ultraviolet light and development with water and capable of giving a patterned layer having excellent adhesion to the substrate surface to serve satisfactorily as a solder mask on printed circuit boards.
A solder mask composition is a resin-based composition used for forming a protective layer of the resin over the whole surface of a printed circuit board and the like excepting the areas where various electronic devices are bonded by soldering in the manufacture of an electronic circuit unit on the printed circuit board. Namely, a solder mask serves to prevent deposition of the solder alloy on to the areas to be protected from contamination with the solder alloy in conducting mounting of electronic devices by soldering to the printed circuit board and also to protect the surface coated therewith from corrosion by the exposure to atmospheric oxygen and moisture.
A solder mask composition is applied pattern-wise to a printed circuit board usually by the method of screen printing and the masking layer is cured by the irradiation with ultraviolet light or by heating. Along with the trend in recent years toward a higher and higher density of the circuit pattern on printed circuit boards, masking layers are also required to be improved relative to the resolution and precision of patterning.
The method of screen printing as a patterning means of a solder mask layer has a limitation in respect of the resolution and fidelity of the pattern not to satisfy the above mentioned requirement for patterning of a solder mask layer in recent years. Accordingly, the method of screen printing is now under continuous replacement with a photolithographic patterning method by using a photopolymerizable resin composition which is superior to the screen printing method in respect of the precision in positioning and reliableness of coating along the edge lines of the conductive circuit lines. Various kinds of photopolymerizable resin compositions have been proposed for use as such a photolithographic solder mask composition including a liquid composition comprising a bisphenol-type epoxy acrylate, photosensitizer, epoxy compound, epoxy curing agent and organic solvent (see Japanese Patent Kokai No. 50-144431 and Japanese Patent Publication No. 51-40451).
A typical procedure for forming a patterned solder mask layer on a printed circuit board by using these photopolymerizable resin compositions is as follows. That is, a printed circuit board is coated all over the surface with the photopolymerizable resin composition in the form of a solution and the coating layer is dried by evaporating the organic solvent in the composition to form a masking layer which is exposed pattern-wise to light to form a latent image of the pattern followed by a development treatment by dissolving away the masking layer on the unexposed areas by using a liquid developer. The liquid developer conventionally used here is an organic solvent in most cases so that serious problems are unavoidable in this process due to the use of a large volume of an organic solvent as the liquid developer in addition to the problems due to the organic solvent contained in the liquid resin composition such as the danger of fire and adverse influences on the workers' health as well as environmental pollution. This is the reason for the strict restrictive regulation as a trend on the use of solution-type solder mask compositions.
In view of the above described problems, photopolymerizable resin compositions of another type suitable as a solder mask have been developed. Namely, photopolymerizable resin compositions, which can be developed with a dilute aqueous alkaline solution, have been developed including, for example, those comprising an alkali-soluble resin such as a reaction product of a novolac-type epoxy compound and an unsaturated monobasic carboxylic acid after a reaction with polybasic acid anhydride, a photopolymerization initiator, a diluent and an epoxy compound as is disclosed in Japanese Patent Publication No. 1-54390, those comprising a copolymer of a (meth)acrylic acid ester and (meth)acrylic acid modified by a (meth)acrylic acid ester having a terminal epoxy group by the addition reaction to a part of the carboxyl groups in the copolymer, a diluent and a photopolymerization initiator disclosed in Japanese Patent Kokai No. 3-172301, those comprising a novolac-type resin and an onium compound disclosed in Japanese Patent Kokai No. 60-175046 and No. 3-154059, and so on.
These prior art photopolymerizable resin compositions are imparted with developability by using an aqueous alkaline solution by the introduction of carboxyl groups into the photopolymerizable ingredient or by the use of an onium-type photopolymerizable ingredient. These photopolymerizable resin compositions, however, are still not free from the problems due to the use of an organic solvent because the resin composition must be in the form of a solution with an organic solvent in order to have an adequate viscosity suitable for application to the surface of a printed circuit board. In addition, a problem is unavoidable there in respect of disposal of the waste of the aqueous alkaline developer solution so that these alkali-developable masking compositions also must be under restriction when they are to be used in a large scale due to the problems to be solved.